1. Field of the Invention
This invention relates to electrical filters employing a mechanical transducer resonator.
2. Description of Related Art
The need to reduce the cost and size of electronic equipment has led to a continuing need forever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units. One class of filter element that meets these needs is constructed from mechanical resonators such as acoustic resonators. See for example, U.S. Pat. No. 5,910,756 issued Jun. 8, 1999 to Ella.
These devices use acoustic waves, for example, bulk longitudinal waves in thin film material, typically but not exclusively piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The resonator sandwich may be suspended in air, supported along its rim, or may be placed on an acoustic mirror comprised of a plurality of alternating layers of high and low acoustic impedance (the product of speed and density), usually silicon dioxide and aluminum nitride, respectively.
When an electric field is applied between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. For certain crystal orientations, such as having the c axis parallel to the thickness of an Aluminum Nitride film, the sound waves propagate in the same direction as the electric field and reflect off of the electrode/air or electrode/mirror interface.
At a certain frequency which is a function of the resonator thickness the forward and returning waves add constructively to produce a mechanical resonance and because of the coupling between mechanical strain and charge produced at the surface of a piezoelectric material, the device behaves as an electronic resonator. The fundamental mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the piezoelectric plus electrode layers. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be more compact than dielectric cavity resonators. Resonators for 50 Ohm matched applications in the GHz range may be constructed with physical dimensions approximately 100 micrometers in diameter and few micrometers in thickness.
Combinations of such resonators may be used to produce complex filters for band pass applications as disclosed inter alia in the aforementioned U.S. Pat. No. 5,910,756 issued to Ella. This patent describes the use of multiple acoustic resonators in constructing ladder and T type band pass filters. The resonant frequency of the resonator is a function of the acoustic path of the resonator. The acoustic path is determined by the distances between the outer surfaces of the electrodes. When batch producing resonators on a substrate, the thickness of the transducer material and the electrodes is fixed at fabrication; hence, the resultant resonance frequency is also fixed. To change the resonant frequency, material may be added to resonator to increase its thickness.
In manufacturing filters that include a multiplicity of resonators such as a T cell type filter, wherein two resonators have a first resonant frequency and the third has a different resonant frequency, it is often convenient to first produce all three resonators with a single resonant frequency, and add material to one of the three to shift its resonant frequency. This method is not, however without problems. For example, in cases where it is desired to fabricate xe2x80x9cT-cellxe2x80x9d filters requiring multiple resonators with different resonant frequencies, but on the same substrate, the material for purposes of frequency shifting is often deposited as a continuous layer over all the resonators. This continuous layer is then patterned to leave the desired added material on the one, usually the uppermost, resonator electrode.
While this technique might appear to be straightforward and easy, it is difficult to precisely pattern an added layer to correspond exactly to an underlying previously patterned electrode. A slight shift in the mask results in the creation of a resonator having three regions of differing resonant frequencies as shown in FIG. 3. As illustrated, there is a first region 31 where the electrode is uncovered by the added material, a region 33 where the added material covers the rest of the electrode and a third region 35 where the added material is over the transducer but outside the electrode area. Such structure is undesirable as it introduces parasitic resonance(s) which degrade the filter performance.
There is thus still a need for an improved process to accurately and predictably shift the resonant frequency of resonators by the addition of material, advantageously a process that does not require extremely accurate patterning of the added material.
There is therefore provided, in accordance with the present invention, a method for adjusting an electromechanical resonator resonant frequency by increasing the total thickness of the resonator which produces a resonator with substantially perfectly aligned resonator layers, thereby avoiding the problems of misaligned layers discussed above.
The simplest resonator form is a sandwich of three layers, a first layer being conductive forming the bottom electrode, an intermediate layer of a transducer material and another conductive layer forming the top electrode. The resonant frequency of this resonator structure may be adjusted by the addition of material over any one of the three layers, most commonly the top electrode which is exposed and readily accessible.
According to the present invention, additional, frequency adjusting material is deposited over the top electrode of a resonator structure and etched so as to form coextensive frequency adjusting and top electrode layers as follows: First, there is formed a patterned area of the frequency adjusting material over the top electrode. Next, the patterned area is masked with a mask having a mask area smaller than the patterned area and being fully contained within the patterned area. After this masking, any material not covered by the mask is removed by etching. The mask remains in place through the etching of both the frequency adjusting material and the top electrode.
This process is particularly useful in cases where more than one resonators are produced side by side and which are later interconnected to form electronic filters. Such resonators are typically all produced simultaneously and all have substantially the same thickness and therefore the same resonant frequency. Using the above process, the three resonator layers are formed as continuous layers. Next a frequency adjusting material is patterned over selected areas of the topmost of the three layers in areas where it is desired to form resonators having a frequency other than the frequency resulting from the three layers alone. Next, masks are placed over both the patterned areas, again fully contained within the patterned areas, and masks are placed outside the patterned areas. Following a subsequent etching step in which the unmasked material is removed, there are simultaneously produced resonator structures of two different resonant frequencies. Again because the masks remain stationary during the etching step, the frequency adjusting layer and the top electrode of the resonator having the frequency adjusting layer form coextensive layers. Therefore, the resulting resonator structure does not exhibit the parasitic resonant frequencies that are encountered in structures produced using the prior art processes.
The above process is not limited to adding frequency adjusting material only to the topmost layer of the resonator structure, but such material may be added to the other layer by altering the order of processing the layers. Thus added material may be placed over the bottom electrode and made coextensive with the transducer layer, under the bottom electrode and made coextensive with the bottom electrode or over the transducer layer and made coextensive with a later deposited top electrode.